This invention relates to gyroscopes, and more particularly to dynamically tuned free rotor gyroscopes.
A dynamically tuned free rotor gyroscope, or gyro, includes a case, a drive shaft and associated motor for rotating the drive shaft on a set of bearings with respect to the case about a drive shaft spin axis, and a hinge supported rotor adapted for rotation about a rotor spin axis. Classically, the rotor is attached to the drive shaft by way of a universal hinge having a single gimbal element with elastic bending or torsion pivot elements. The gimbal is pivoted about two mutually orthogonal axes that intersect at a pivot point coincident with both the rotor spin axis and the drive shaft spin axis. The gimbal is attached to a rotor along one pivot axis called the gimbal-rotor pivot axis. The drive shaft is attached to the gimbal along the orthogonal pivot axis called the gimbal-shaft axis. The gimbal-rotor pivot axis is orthogonal to the rotor spin axis and the gimbal shaft-pivot axis is orthogonal to the shaft spin axis.
In operation, as the motor drives the drive shaft, and thus also the rotor, the rotor precesses due to torques that are proportional to the angular deviation of the rotor spin axis from a force-free equilibrium position aligned with the rotor drive shaft axis. These torques are either directly proportional to the above-mentioned angular deviation, or are proportional to the mentioned angular deviation modulated sinusoidally at two-times spin speed. These torques are herein referred to as direct elastic restraint torques, and as anisoelastic restraint torques, respectively. Generally the torques are produced by the bending of the elastic suspension pivots and the spinning, oscillating gimbal.
With this configuration, there is a shaft spin speed called the tuned speed at which the gyroscope rotor precession due to the sum of the direct elastic restraint torques vanishes. At this tuned speed, for small angular deviations, the direct elastic restraint torques acting on the rotor (which are due to elastic pivots and other in-phase means) are cancelled by inertial torques caused by the dynamics of the constrained spinning, oscillating gimbal. See E. W. Howe and P. H. Savet, "The dynamically tuned free rotor gyro," control engineering, PP. 67-72, June 1964.
In the prior art, the dynamic tuning by spin speed adjustment may be augmented by the following methods:
(a) adjusting the bending or torsional stiffness of the pivots, PA1 (b) adjusting the mass distribution of the gimbal along the axis perpendicular to the plane of the orthogonal pivot axes, PA1 (c) adjusting the spin speed of the drive shaft, PA1 (d) applying a feedback torque proportional to the rotor hang-off angle. PA1 (a) adjusting the relative bending or torsional stiffness of the pivots, or PA1 (b) adjusting the relative mass distribution, of the gimbals along the axis perpendicular to the plane of the pivot axes.
However, once a gyroscope is fabricated and assembled, only the spin speed adjustment and feedback torque methods are readily available for tuning the rotor precession.
It is well known in the prior art that a gyro having a hinge with a single gimbal is susceptible to errors produced by shaft angular motion near twice the spin frequency. See, for example, R. J. B. Craig, "Theory of Errors in a Multi-Gimbal Elastically Supported, Tuned Gyroscope," IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-8, No. 3, pp. 289-297, May, 1972, and U.S. Pat. No. 3,678,764. Typically, these anisoelastic effects are less than 0.1 Newtonmeter per radian of shaft angular vibration referenced to the rotor spin axis. Shaft angular vibrations may be caused by bearing inhomogenieties, or by vibrations of the gyroscope housing, or case, transmitted to the shaft. In a relatively quiet dynamic environments, the bearing angular motions may be relatively uniform over a short span of time. In this case, the average error torques transmitted to the rotor may be nearly constant and thus appear as a fixed error torque that can be calibrated and compensated. However, changes in the steady state bearing second harmonic angular motion appears as a bias change in the error torque experienced by the rotor. Furthermore, in dynamic environments, a shaft angular motion may change substantially, giving rise to changing error torques transmitted to the rotor.
In the prior art, these transmitted error torques are nulled by using a plurality of gimbals in the universal hinge. These gimbals and their pivot axes are arranged in such a manner that a cancellation of errors produced by the shaft angular motion at twice spin frequency occurs. This cancellation may be achieved by methods of either:
However, once a prior art gyroscope is fabricated and assembled, it is prohibitively difficult to adjust the pivot stiffness or gimbal mass distribution. Consequently, in the prior art, these anisoelastic restraint adjustments must be made before assembly or those adjustments must be ignored altogether.
In the prior art, gyroscopes typically utilize a plurality of universal hinge elements with mechanical means for adjusting gimbal mass distribution on the spin axis, or alternatively utilize exceptional care in machine tolerancing the pivot dimensions. The absolute pivot dimension must typically be within a fraction of 10.sup.-6 meter in order that the pivot stiffness be within a percent of a nominal value. Where parallel gimbals are used, for example, as in U.S. Pat. Nos. 3,678,764 and 4,143,451, extreme care must be taken to align the pivot axis of one gimbal with respect to another so that misaligned pivots will not contribute to excessive stiffness of the combined pivot joints. The precision balancing and adjustment procedures are expensive and must be done before assembly of the gyroscope. Furthermore, after assembly, no further tuning or matching of mechanical parameters is possible). For these reasons, in the prior art, it is not possible to either fine tune the direct elastic restraint torques or to match the anisoelastic restraint torques after gyroscope assembly.
There are also other sources of elastic restraint, such as viscous drag on the rotor, signal generator field energy, motor winding fields, and others that cannot properly be tuned before assembly. Furthermore, with the aging of the gyroscope, the effective elastic restraint changes so that the free rotor behavior is often degraded with time. In addition, bearings have to be carefully selected for homogeniety to minimize shaft vibration. Typically this vibration cannot be controlled through dynamic environments and with natural aging.
The error mechanisms that particularly plague dynamically tuned free rotor gyroscopes are those error torques that are proportional to the bending of the hinge flexures from a position aligned by very careful, accurate and precise matching, balancing and assembly of the hinge suspension.
Generally, this error mechanism is addressed by "matching" flexure bending compliance with gimbal inertias. This is accomplished by careful prematching of flexure dimensions or by adjusting gimbal axis mass moments. The disadvantage of such gimbal adjustment scheme, is that flexures must be machined to a width tolerance of about five percent to bring compliance within bounds that enable tuning by adjustment of gimbal weights. However, to attain hinge rigidity with respect to linear vibrations, it is necessary to have short flexure blades. This in turn demands thin flexure blades to obtain low bending compliance. Therefore a five percent flexure width tolerance typically implies about one micrometer tolerance on flexure absolute width machining. This tolerance is achievable only with expensive and time consuming machining. If no gimbal inertia adjustment is used, flexure width tolerances become more stringent, usually to less than 0.1 micrometer. This is only achievable by combination of lapping and careful measuring of bending compliance.
Accordingly, it is an object of the present invention to provide a system for tuning a gyroscope having a universal hinge, or a plurality of universal hinges, by electromechanical means.
It is another object to provide an electromechanical system for suppressing 2N rectification torques in a free rotor gyroscope.